1. Field of the Invention
This invention relates to a sensing apparatus and more particularly to an apparatus for sensing the presence of an inductive load driven by a pulse width modulated (PWM) signal.
2. Description of the Related Art
Media storage systems are well known in the art and are commonly used to store data cartridges at known locations and to retrieve desired data cartridges so that data may be written to or read from the data cartridges. Such media storage systems are often referred to as an autochanger media storage system or, simply, autochanger (“media storage system”).
A typical media storage system can include a slot for holding a single data cartridge, or cartridge storage racks or “magazines” that hold several data cartridges. Another type of data cartridge holding device is a cartridge read/write device for reading data from or writing data to the data cartridges within the media storage system. The cartridge storage racks can be accessed by a system operator by opening drawers in the front of the media storage system housing to reveal the cartridge storage racks.
A typical media storage system is also provided with a cartridge handling system for transporting the data cartridges between the cartridge racks and the cartridge read/write device. A typical cartridge handling system includes a cartridge engaging assembly or “picker” for engaging the data cartridges and a positioning device for moving the cartridge engaging assembly between the racks and the read/write device.
Media storage systems of the type described above are usually connected to a host computer system, which can access or store data on the data cartridges. A control system associated with the autochanger actuates the positioning system to move the picker along the cartridge storage locations until the picker is positioned adjacent the desired data cartridge. The picker can then remove the data cartridge from the cartridge rack and carry it to the cartridge read/write device. Once properly positioned adjacent the cartridge read/write device, the picker can insert the selected data cartridge into the cartridge read/write device so that the host computer can read data from or write data to the data cartridge. After the read/write operation is complete, the picker can remove the data cartridge from the cartridge read/write device and return it to the appropriate cartridge rack.
One concern with the type of media storage system is that after a drawer is opened by the operator to access the data cartridges, it can be left fully or partially open or not properly closed. If the picker attempts to remove a data cartridge from a rack in an open or improperly closed drawer, the picker can be damaged. To help prevent this damage, the media storage system is equipped with a solenoid adjacent to each of its drawers to lock the drawers in the closed position prior to operation of the picker. Each drawer has a funnel receiver arranged adjacent to its solenoid so that when the solenoid is activated the plunger engages the funnel receiver to lock the drawer in the closed position. When the solenoid is deactivated it disengages the funnel receiver and the drawer is free to open.
One disadvantage of this type of system is that one or more of the solenoids can be disconnected from its power connection during assembly of the media storage system or through use. If the solenoid is not connected to its power connection it will not lock its drawer. One way to test for this disconnect condition is for the operator to manually pull each one of the drawers to be sure they are locked after the solenoid activate signal is given. This method of testing is inconvenient and time-consuming. Another way to verify that the solenoids are present and connected properly is to provide an apparatus that applies an AC waveform to drive the solenoid and then monitors the resulting waveform. While effective, this approach is complex and expensive.